// 背景图
bg-img($url)
  background-image: url($url + '@2x.png')
  @media (-webkit-min-device-pixel-ratio: 3), (min-device-pixel-ratio: 3)
    background-image: url($url + '@3x.png')

// 1px边框
bb1($color)
  position relative
  &:after
    display block
    position absolute
    left 0
    bottom 0
    width 100%
    content ''
    border-top 1px solid $color
    
@media (-webkit-min-device-pixel-ratio: 1.5), (min-device-pixel-ratio: 1.5)  
  .bb1
    &::after  
      transform: scaleY(0.7)  
  
@media (-webkit-min-device-pixel-ratio: 2), (min-device-pixel-ratio: 2)  
  .bb1
    &::after  
      transform: scaleY(0.5)